Many years have passed since diagnostic imaging modalities including X-ray computed tomography (CT), magnetic resonance imaging (MRI), and diagnostic ultrasound became indispensable in medical practices. The modalities are designed to image differences in a CT value in a tissue, differences in a nuclear spin relaxation time, or differences in acoustic impedance. Since the differences in a physical nature reflect the structure (shape) of a tissue, the imaging is referred to as morphological imaging. In contrast, imaging of a region in a tissue that is structurally identical to the other region therein but is functionally different therefrom is referred to as functional imaging.
In recent years, visualization of the state of the brain or a tumorous region through positron emission tomography (PET) has especially attracted attention. The PET is a technique of handling radioactively metabolized molecules so as to perform functional imaging on the level of molecules. In contrast, as functional imaging to be performed on the level of tissues, there is elasticity imaging that is a technique of imaging differences in hardness in a tissue. This technique is intended to acquire information, which is supposed to be acquired through physician's palpation, using a diagnostic system. As a lump leads to early diagnosis of breast cancer, so hardness becomes a significant factor that reflects a cancerous tissue or the like. Supposing the hardness of a microscopic region can be imaged using a diagnostic system, arteriosclerosis can be examined or a pathology that cannot be revealed by palpation can be diagnosed.
Hardness to be examined by palpation is represented by a modulus of rigidity (a modulus of shear elasticity). For imaging of moduli of elasticity, a technique is often adopted that: an operator presses a probe against a body surface; and a local deformation (distortion) factor of an intracorporeal tissue is calculated in order to detect a hardness distribution. The modulus of shear elasticity is one of physical quantities that are hard to accurately measure. Moreover, elasticity imaging proves its worth in imaging of an early-phase lesion that is hard to distinguish through normal diagnostic imaging. Therefore, relative moduli of elasticity other than absolute moduli of elasticity are calculated in order to visualize a lesion. The diagnosis based on the relative moduli of elasticity has become a mainstream in clinical practices.
The elasticity imaging provides an unprecedented diagnostic technique. For prevalence of the technique, in addition to training of an operator and demonstration or discussion of the technique, a tissue mimicking phantom is needed.
A conventionally known phantom for elasticity imaging is based on an existing phantom designed for normal ultrasonic echography. The fundamental structure of the phantom is such that graphite or any other powder is mixed in a gel of a polymer such as agar or gelatin. The gel has solvent molecules bound in a macromolecular network, and is apparently solid. A hydro-gel prepared by adopting water as a solvent has the same acoustic property as water and a soft tissue of a tissue. Acoustically, the hydro-gel can substantially be regarded as a simulation of a tissue. Since the hardness of the gel can be readily controlled by changing a macromolecular concentration or any other condition for production, the hydro-gel is an excellent material for an elasticity imaging phantom.
In general, a macromolecular strand included in a hydro-gel and water are hardly different from each other in terms of acoustic impedance. The use of a gel alone cannot sufficiently produce ultrasonic echoes. Therefore, as mentioned above, a powder such as graphite whose acoustic impedance is different from the acoustic impedance of water is mixed in the gel. Ultrasonic echoes to be returned from the entire gel can be controlled by adjusting ultrasonic echoes to be returned from the interface between the powder and water. Based on this idea, a phantom is produced as mentioned in, for example, “1996 IEEE Ultrasonic Symposium” (p. 1502-1505).
Moreover, a gel of agar or gelatin is referred to as a thermally-reversible gel, and reversibly changes between a sol (highly fluid state) and the gel (less fluid state). The gel whose states change with temperature has macromolecular networks thereof bonded relatively loosely and is therefore mechanically less strong.
As a solution, a method using a polyvinyl alcohol gel that exhibits low thermal reversibility despite its preparation including steps of heating and cooling has been proposed as described in Japanese Patent Application Laid-Open No. 8-10254.